Thermosensitive decolorable ink compositions prepared by making use of a developing and decoloring mechanism of a leuco dye have so far been known in large numbers.
Known are, for example, a decolorable or discolorable aqueous ink characterized by containing a colorant for an aqueous ink which is decolorable by a solvent or heating, wherein the colorant comprises fine powder comprising at least a leuco dye, developer, a decolorant and a resin in which either lower temperature out of a glass transition temperature or a Vicat softening temperature of the resin described above is higher than 50° C. and in which a melting point or a moldable temperature thereof is 230° C. or lower (refer to, for example, patent document 1) or a colorant prepared by coloring spherical fine particles made of a resin by a coloring substance selected from leuco dyes (refer to, for example, patent document 2).
However, in the aqueous inks shown in patent document 1 and patent document 2, the problem of color fading of the colorants with the passage of time is solved, and written or printed characters or images are decolored by a solvent such as acetone or with heating at 200° C. (an iron in the examples). Accordingly, they can not be readily decolored with heating by rubbing and the like, and problems are involved therein in terms of usability and versatility.
On the other hand, known as thermosensitive decolorable colorants prepared by using leuco dyes which can be decolored with heating by rubbing and the like are, for example, temperature-sensitive discolorable color-memory microcapsule pigments which constitute a homogeneous compatibilized composite comprising as essential components, three components of (a) an electron-donating coloring organic compound (a leuco dye, hereinafter the same shall apply), (b) an electron-accepting compound (a developer, hereinafter the same shall apply) and (c) specific esters and specific aryl alkyl ketones controlling coloring reactions of (a) and (b) described above and which show large hysteresis widths (ΔH) of 8° C. to 30° C. and 8° C. to 80° C. in a color intensity-temperature curve to exert a quasi-reversible heat discoloration characteristic (refer to, for example, patent documents 3 and 4).
Further, known as heat erasable writing instruments prepared by using the above microcapsule pigments are, for example, a heat erasable writing instrument filled with a temperature-sensitive discolorable color-memory ink composition which comprises at least a solvent, a resin and a microcapsule pigment containing therein a temperature-sensitive discolorable color-memory composition comprising a homogeneous comptabilized composite of (a) an electron-donating coloring organic compound, (b) an electron-accepting compound and (c) a reaction medium controlling a coloring reaction of (a) and (b) described above, wherein the microcapsule pigment described above shows a hysteresis width (ΔH) of 40° C. to 70° C. in a color intensity-temperature curve and which ink composition has a complete decoloration temperature (T4) of 45 to 95° C. and a color developing initiation temperature (T2) of 0° C. or lower and is changed in color from color to non-color (refer to, for example, patent document 5) and ball point pen type writing instruments filled with an aqueous ink composition for a reversible heat discolorable writing instrument which contains at least water and a reversible heat discolorable microcapsule pigment containing therein a reversible heat discolorable composition comprising (a) an electron-donating coloring organic compound, (b) an electron-accepting compound and (c) a reaction medium determining a temperature of initiating a coloring reaction of both compounds described above, wherein an average particle diameter of the reversible heat discolorable microcapsule pigment described above falls in a range of 2.5 to 4.0 μm and in which the particles having a particle diameter of 2.0 μm or less account for 30% by volume or less based on the whole microcapsule pigment and wherein the reversible heat discolorable microcapsule pigment described above has a complete color-developing temperature (t1) of −30 to 0° C. and a complete decoloration temperature (t4) of 50 to 95° C. in a color intensity-temperature curve (refer to, for example, patent document 6).
The thermosensitive decolorable ink compositions described in patent documents 3 to 6 described above contain in a microcapsule, a leuco dye which is an electron-donating coloring organic compound, a developer which is an electron-accepting compound, a crystalline substance and the like. In the above decoloration mechanism, the leuco dye and the developer are first dissolved in the molten crystalline substance, and the molten crystalline substance is crystallized with cooling to deposit the leuco dye and the developer. The leuco dye and the developer which are not in a molten state are interacted (neutralized) with each other to develop a color by opening of a lactone ring and the like in the leuco dye, and a toner staying in a color developing state is obtained. In the above toner staying in a color developing state, the crystalline substance is molten with heating by rubbing and the like to dissolve again the leuco dye and the developer, and therefore the leuco dye and the developer can not be interacted and stay in a decoloring state.
When heating by rubbing and the like is stopped, the toner is cooled down to room temperature, and the crystalline substance is crystallized; the leuco dye and the developer are deposited again and interacted, and they stay in a color redeveloping state. In order to prevent the above state, a substance generating thermal hysteresis is mixed with the crystalline substance to inhibit recrystallization thereof at room temperature. A melting point of the crystalline substance in the thermosensitive toner is room temperature or higher (for example, 60° C.), and the freezing point in recrystallization after melting is set to room temperature or lower (for example, −10° C.) to thereby prevent color redeveloping by cooling to room temperature.
Accordingly, the leuco dyes described in patent documents 3 to 6 described above stay in a decoloring state when the crystalline substance is molten and stay in a color developing state when it is solidified, and therefore the decoloring and color developing states are controlled by controlling (thermal hysteresis) melting and solidifying of the crystalline substance in conventional techniques.
In conventional thermosensitive decolorable ink compositions, however, a mechanism of controlling decoloring and color developing is to expand a thermal hysteresis of the crystalline substance, and therefore the problem that decolored drawn lines stored at a resolidifying temperature or lower of the crystalline substance are developed again in color is involved therein.
Also, color redeveloping can be prevented by expanding a width of thermal hysteresis, but the problem that color redeveloping at a lower temperature state can not be prevented is involved therein because effective substances are not found in the existing state and the crystalline substance is crystallized at some stage.
Further, in order to provide the crystalline substance with the thermal hysteresis described above, impurities have to be prevented from being mixed therein, and it is essential to microcapsulize the crystalline substance. At present, it is difficult to achieve microcapsulization with an average particle diameter of 20 μm or less, or even if the microcapsule having an average particle diameter of 20 μm or less can be produced, the production efficiency thereof is low. If the microcapsule having a large average particle diameter is used, the hue intensity is low or an amount of the crystalline substance molten for erasing drawn lines is increased, and therefore the problem that more energy is required for decoloration is involved therein.    Patent document 1: Japanese Patent Application Laid-Open No. 2001-271011 (claims, examples and others)    Patent document 2: Japanese Patent Application Laid-Open No. 2002-294104 (claims, examples and others)    Patent document 3: Japanese Patent Application Laid-Open Hei 7 No. 33997 (claims, examples and others)    Patent document 4: Japanese Patent Application Laid-Open Hei 8 No. 39936 (claims, examples and others)    Patent document 5: Japanese Patent Application Laid-Open No. 2006-63238 (claims, examples and others)    Patent document 6: Japanese Patent Application Laid-Open No. 2008-280440 (claims, examples and others)